New triple tungstate phosphors NaPbLa(WO4)3:Yb3+/Ho3+ (x = Yb3+/Ho3+ = 7, 8, 9, 10) are successfully fabricated by microwave assisted sol-gel synthesis and their structural and frequency upconversion (UC) characteristics are investigated. The compounds crystallized in the tetragonal space group I41/a and the NaPbLa(WO4)3 host have unit cell parameters a = 5.3927(1) and c = 11.7961(3) Å, V = 343.05(2) Å3, Z = 4. Under excitation at 980 nm, the phosphors have yellowish green emissions, which are derived from the intense 5S2/ 5F4 → 5I8 transitions of Ho3+ ions in the green spectral range and strong 5F5 → 5I8 transitions in the red spectral range. The optimal Yb3+:Ho3+ ratio is revealed to be x = 9, which is attributed to the quenching effect of Ho3+ ions, as indicated by the composition dependence. The UC characteristics are evaluated in detail under consideration of the pump power dependence and Commission Internationale de L'Eclairage chromaticity. The spectroscopic features of Raman spectra are discussed in terms of the superposition of Ho3+ luminescence and vibrational lines. The possibility of controlling the spectral distribution of UC luminescence by the chemical content of tungstate hosts is demonstrated.

Abstract
1. Introduction
2. Experimental Procedure
3. Results and Discussion
4. Conclusions
References

• Chang Sung Lim(Department of Aerospace Advanced Materials & Chemical Engineering, Hanseo University) Corresponding author
• Victor V. Atuchin(Laboratory of Optical Materials and Structures, Institute of Semiconductor Physics, SB RAS/Laboratory of Semiconductor and Dielectric Materials, Novosibirsk State University/Research and Development Department, Kemerovo State University)
• Aleksandr S. Aleksandrovsky(Laboratory of Coherent Optics, Kirensky Institute of Physics Federal Research Center KSC SB RAS/Department of Photonics and Laser Technologies, Siberian Federal University)
• Yuriy G. Denisenko(Department of Inorganic and Physical Chemistry, Tyumen State University/Department of General and Special Chemistry, Industrial University of Tyumen)
• Maxim S. Molokeev(Laboratory of Crystal Physics, Kirensky Institute of Physics, Federal Research Center KSC SB RAS/Siberian Federal University/Department of Physics, Far Eastern State Transport University)
• Aleksandr S. Oreshonkov(Siberian Federal University/Laboratory of Molecular Spectroscopy, Kirensky Institute of Physics Federal Research Center KSC SB RAS)